Energy storage requirements continue to grow as the electronic, portable power, and energy infrastructure industries expand and transition away from more historic non-renewable energy supplies. For example, there has been a renewed interest in batteries and other energy storage devices for use in electric and hybrid automobiles, and this has been caused, in part, by volatile oil costs and the possibility of catastrophic climate change that has greatly pushed scientific attention toward the development of electrical and hybrid vehicles powered by rechargeable batteries, e.g., rechargeable lithium-ion (Li-ion) batteries that may be powered with electricity from renewable sources. Similarly, there is ongoing research in ways to make lighter and more efficient batteries for electronic devices ranging from portable computers to cellular phones and other wireless communication devices.
General goals for battery manufacturers include providing long life and significant power levels with the least amount of weight while also providing a recharging functionality. More specifically, one of the most critical parameters for new energy storage technologies and designs is the demand for higher energy densities (i.e., energy storage per unit of battery or storage device weight). Additionally, there is growing concern over potential long term environmental impacts of product manufacture and use, and, the energy storage industry continues to search for storage devices that can make use of environmentally benign or green materials while still providing desirable energy densities. Unfortunately, many existing electrode materials that have high durable capacities and good rate capability are expensive and/or are toxic. Furthermore, improved energy density and rate capabilities are still demanded by the battery and other energy storage industries such as for battery designs facilitating a successful deployment of a fleet of electric vehicles. Hence, there remains a need for electrodes fabricated from abundant and nontoxic elements with durable high-reversible capacity and highly improved rate capability.
In the search for electrode materials for electrochemical devices such as batteries, smart windows, and the like, many efforts have centered on materials with structures that can intercalate small cations without major structural changes occurring. For example, lithium-on batteries are one of the most prevalent energy storage devices for portable electronics and for vehicles because these batteries offer relatively high energy densities and longer lifespans than comparable technologies. Lithium-ion batteries utilizing existing technologies and electrode design have sufficient specific energy and power densities to meet some targets for hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs) for up to a 40 mile range.
However, significant improvements in lifetimes of batteries along with reductions in costs and use of less toxic electrode materials are needed before lithium-ion batteries are employed fully in the transportation industry. Increasing energy density of electrode materials, for example, is desirable to support use of electrochemical devices such as lithium-ion batteries being used in fully electric vehicles. Note, also, that materials for electrochromic applications and devices are required to meet many of the same criteria as called for in batteries, and the following description may use the word electrochemical device to apply to nearly any electric device with an electrode such as a battery or an electrochromic device.
With reference to some exemplary electrode research or design efforts, three-d-transition metal oxides (Fe2O3, Fe3O4, MOo3, CoO, NiO, and the like) are capable of Li+ insertion/extraction in excess of 6 Li+ per formula unit, resulting in a larger reversible capacity than commercially employed graphite. For example, the specific capacity of metal oxide anodes can be over 1000 mAh/g, which is approximately three times higher than that of graphitic carbons. Differing from the intercalation mechanism occurring with graphite, the 3d transition metal oxides are reduced in a conversion reaction to small metal clusters, and the oxygen reacts with the lithium to form Li2O. In general, this leads to volumetric expansion and destruction of the structure upon electrochemical cycling, which, for bulk particles, typically results in capacity loss during cycling, even at very low rates.
It has also been reported, for example, that MoO3 nanoparticles that react with approximately 5.7 Li ions may lead to an electrode with a durable reversible capacity as high as 1050 mAh/g. Additionally, an Fe3O4-based Cu nano-architectured electrode has been developed that allowed for small diffusion paths and better electrical and mechanical contact by using a Cu-nanopillar current collector, enabling improved rate capability. Various groups have also reported the use of metal oxides with optimal sizes and carbon nanostructures or nanostructures with carbon-modified surfaces to improve reversible capacity and rate capability. Highly dispersed Fe3O4 nanocrystals have been used in a carbon matrix that provided an electrode that had a reversible capacity of about 600 mAh/g at 0.1C rate. “C” represents “charge rate” signifying a charge or discharge rate equal to the capacity of a battery divided by one (1) hour. Further studies have shown electrodes formed with carbon/Fe3O4 composite nanofibers fabricated with an electro-spinning technique had a reversible capacity of 1007 mAh/g at 0.1C and 623 mAh/g at 2C rate. While these efforts have shown improvements in electrode technologies, these designs have not been widely adopted as there remains a need for even higher energy densities and other improvements in electrodes before such electrodes will be implemented by the transportation and other industries. For example, electrodes formed of more green materials are needed with high reversible capacities and improved rate capabilities as well as desirable energy densities.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.